1. Field of the Invention
The invention relates to a radar system that detects a target and a target direction detecting method.
2. Description of the Related Art
There is known an FM-CW or phase-monopulse radar system as a radar system that detects a target. Japanese Patent Application Publication No. 2000-147102 (JP-A-2000-147102) describes an FM-CW radar system. In addition, JP-A-2000-147102 describes that it is determined whether the neighborhood sum of a spectrum peak is smaller than or equal to a threshold and it is determined whether a moving object prediction flag is set, and, when the neighborhood sum is smaller than or equal to the threshold and the moving object prediction flag is not set, it is determined that the spectrum peak is the spectrum peak of a stationary object.
In addition, Japanese Patent Application Publication No. 2004-340755 (JP-A-2004-340755) describes a phase-monopulse radar system. In JP-A-2004-340755, a radar wave formed of a frequency increasing portion, a frequency decreasing portion and a frequency constant portion is transmitted, a reflected wave of the radar wave is received by two antennas, and, for each of receiving signals, a beat signal that indicates a difference in frequency between the transmitting signal and the receiving signal is generated for each of the frequency increasing portion, the frequency decreasing portion and the frequency constant portion. Then, on the basis of a phase difference between the beat signals at the respective peak frequencies of the frequency increasing portion beat signals and a phase difference between the beat signals at the respective peak frequencies of the frequency decreasing portion beat signals, when a pair of the peak frequencies of the frequency increasing portion beat signals and the peak frequencies of the frequency decreasing portion beat signals cannot be matched with each other, it is assumed that peak frequencies resulting from a plurality of objects overlap each other and then the phase of the object is calculated from a phase difference between the beat signals at the respective peak frequencies of the frequency constant portion beat signals.
There is known a phase-monopulse system as one of systems that measure the direction of a target with respect to a radar system. FIG. 1 shows a diagram of the principle of a phase-monopulse system. In the phase-monopulse system, for example, two antennas A1 and A2 are arranged, and the direction of an incoming radio wave is obtained on the basis of a phase difference (Δφ) of signals received respectively by the antennas A1 and A2. The phase difference is expressed by the mathematical expression (1) where the incoming angle is θ, the distance between the antennas is d and the wavelength of a carrier wave (reflected wave) is λ.Δφ=2π·(d·sin θ/λ)  (1)
In calculating the phase difference, first, a transmitted wave modulated with a triangular wave is output from an antenna and then parts of received waves reflected by the target and received by the antennas are mixed with part of the transmitted wave to thereby acquire the frequencies of beat signals. Then, the beat signals are subjected to Fourier transformation to obtain frequency spectrum data, and then the peak frequencies of the respective frequency spectra are detected from the frequency spectrum data. The frequency spectrum data are expressed as complex vectors on a complex plane. Each peak frequency of the detected frequency spectrum is a frequency corresponding to a distance to the target and a relative velocity with respect to the target. Then, when the peak frequencies of the frequency spectra are identified, the phases of the beat signals at the respective peak frequencies are calculated. Here, because the frequency spectrum data may be expressed as complex vectors on a complex plane, the phase of each beat signal may be, for example, calculated from an angle made between the complex vector and the real axis on the complex plane. Then, the difference in phase between the respective beat signals is obtained to calculate the phase difference, and then the direction of the target may be calculated from the calculated phase difference.
Here, in the above described phase-monopulse system, in the case where a plurality of targets are present, if the peak frequencies of frequency spectra corresponding to the respective targets coincide with each other, the directions of the respective targets may not be accurately calculated. FIG. 2 shows an example of the positional relationship between targets and a radar system. More specifically, FIG. 2 shows a situation in which an oncoming vehicle (moving target) is travelling ahead of a vehicle (host vehicle) equipped with the radar system and guard rails (stationary target) each having a plurality of posts are installed on a side over the oncoming vehicle.
Here, FIG. 3A is an example of frequency spectrum data acquired by the radar system shown in FIG. 2. FIG. 3B shows a graph that expresses the frequency spectrum data of FIG. 3A as a complex vector. In FIG. 3A, the abscissa axis represents a frequency, and the ordinate axis represents a reflection level. The solid line in FIG. 3A indicates reflection levels from the stationary target, that is, the guard rails. The four peaks in FIG. 3A correspond to reflected waves from the posts of the guard rails. The dotted line in FIG. 3A indicates a reflection level from the moving target, that is, the oncoming vehicle. In the example of FIG. 3A, the peak frequency of the moving target overlaps with the peak frequency of the stationary target, and the peak values of the reflection levels coincide with each other. If the peak frequencies of the frequency spectra overlap each other in this way, the phases are combined and cannot be separated from each other in the radar system. In other words, the frequency spectrum data originally include a complex vector corresponding to the post of the guard rail and a complex vector corresponding to the oncoming vehicle; however, actually, only a resultant vector of the two complex vectors can be acquired. Note that FIG. 3B shows a complex plane, the X-axis is a real axis, and the Y-axis is an imaginary axis. FIG. 3B shows an actually acquired resultant vector and also shows a complex vector corresponding to a reflection wave from the stationary target and a complex vector corresponding to a reflection wave from the moving target.
In this way, if the peak frequencies of the frequency spectra corresponding to the respective targets coincide with each other, the directions of the respective targets cannot be accurately calculated. In addition, this may lead to a malfunction of a pre-crash safety system (PCS) or non-detection of the targets.